1. Field of the Invention
The invention concerns a circuit arrangement for firing a controlled semiconductor valve, particularly a thyristor (SCR), whose control path is connected to a d-c voltage source by way of the collector-emitter path of a transistor.
2. Description of the Prior Art
A circuit arrangement for firing a controlled semiconductor valve is known, in which a d-c voltage source is connected to the control path of the semiconductor valve via the collector-emitter path of a transistor. The transistor is switched on and off by a control signal, so that the entire d-c voltage of the d-c voltage source is applied to the control path of the semiconductor valve, if required. The transistor functions here as a switching transistor. It can have only two conduction states. The d-c voltage source can be designed so that it obtains the d-c voltage for control from the voltage at the anode-cathode path of the semiconductor valve. Such a d-c voltage source is known, for instance, from the German Pat. No. 1,538,099.
In such a circuit arrangement, the magnitude of the control electrode current which flows into the control path of the semiconductor valve, depends on the magnitude of the d-c voltage of the d-c voltage source. This is also the case in circuit arrangements with a d-c voltage source, in which the d-c voltage delivered is obtained from the voltage at the anode-cathode path of the semiconductor valve to be fired. Therefore, if the d-c voltage varies, the control electrode current varies also. The circuit arrangement must therefore be designed for that lower limit of the control electrode current which still ensures reliable firing. In the time average this means, however, unnecessary energy consumption.
However, the magnitude of the control electrode current depends also on the control electrode characteristic of the semiconductor valve. In the construction of circuit arrangements, the design would therefore have to take into consideration the control electrode characteristic of the semiconductor valve used in every circuit arrangement concerned. In production, however, one seeks to avoid individual adjustments and to achieve cost-saving mass production with identical components.
It is an object of the invention to develop the circuit arrangement mentioned above such that the control electrode current is kept constant independently of the control electrode characteristic of the semiconductor valve and also in the case of fluctuations of the d-c voltage supplied by the d-c voltage source, if present.